1. Field of the Invention
The present invention relates to a battery temperature monitoring circuit and, more particularly, to a small highly accurate battery temperature monitoring circuit.
2. Description of the Related Art
A battery temperature monitoring circuit is, as the name implies, a circuit that monitors the temperature of a battery. Battery temperature monitoring circuits are commonly used with lithium ion batteries because significant safety issues arise when a lithium ion battery is charged while the temperature of the battery is above or below designated temperature levels.
FIG. 1 shows a schematic diagram that illustrates an example of a prior art battery temperature monitoring circuit 100. As shown in FIG. 1, battery temperature monitoring circuit 100 includes a cold voltage comparator 110 with hysteresis that detects a too cold condition, and a hot voltage comparator 112 with hysteresis that detects a too hot condition.
As further shown in FIG. 1, the cold and hot voltage comparators 110 and 112, which are conventional devices, have a number of inputs that, in the present example, include an upper reference voltage input VR1, a lower reference voltage input VR2, a measured voltage input MM, an enable input EN, and a bias current input BC.
In the present example, the upper reference voltage input VR1 of cold voltage comparator 100 is connected to receive an upper cold reference voltage VC1 that represents a cold trip point temperature, while the lower reference voltage input VR2 of cold comparator 100 is connected to receive a lower cold reference voltage VC2.
Similarly, the upper reference voltage input VR1 of hot comparator 112 is connected to receive an upper hot reference voltage VH1, while the lower reference voltage input VR2 of hot comparator 112 is connected to receive a lower hot reference voltage VH2 that represents a hot trip point temperature.
In addition, the measured inputs MM of both the cold and hot voltage comparators 110 and 112 are connected to receive a measured battery temperature voltage VB, while the enable inputs EN are connected to receive an enable voltage VE. The bias current input BC of cold voltage comparator 110 is connected to receive a bias current BI1, while the bias current input BC of hot voltage comparator 110 is connected to receive a bias current BI2. Further, cold voltage comparator 110 has an output that generates a too cold signal VTC, and hot voltage comparator 112 has an output that generates a too hot signal VTH.
As also shown in FIG. 1, battery temperature monitoring circuit 100 further includes a thermistor 114 that is electrically connected to the measured inputs MM of both the cold and hot voltage comparators 110 and 112, and physically connected to a lithium ion battery 116. In addition, thermistor 114 is thermally connected to lithium ion battery 116 so that the temperature of thermistor 114 is substantially the same as the temperature of lithium ion battery 116.
In the present example, thermistor 114 is implemented as a negative temperature coefficient (NTC) thermistor. An NTC thermistor has a resistance that decreases as the temperature of the NTC thermistor increases, and increases as the temperature of the NTC thermistor decreases. Alternately, thermistor 114 can be implemented as a positive temperature coefficient (PTC) thermistor, which has a resistance that increases as the temperature of the PTC thermistor increases, and decreases as the temperature of the PTC thermistor decreases.
Battery temperature monitoring circuit 100 further includes a current source 120 that sources a zero temperature coefficient (0TC) constant current Ito thermistor 114 to generate the measured battery temperature voltage VB. (A 0TC current is a current that has a constant magnitude over changes in temperature.)
In operation, the cold trip point temperature of lithium ion battery 116 is the temperature where lithium ion battery 116 is too cold to be safely charged, and the hot trip point temperature is the temperature where lithium ion battery 116 is too hot to be safely charged. For example, lithium ion battery 116 may require a cold trip point temperature of 0° C. and a hot trip point temperature of 62° C.
In addition, thermistors typically have a look up table with resistances associated with a range of temperatures. Thus, the upper cold reference voltage VC1 is equal to constant current I multiplied times the resistance of thermistor 114 that is associated with the cold trip point temperature, e.g., 0° C.
Similarly, the lower hot reference voltage VH2 is equal to the constant current I multiplied times the resistance of thermistor 114 that is associated with the hot trip point temperature, e.g., 62° C. The lower cold reference voltage VC2 can be set a predefined voltage below the upper cold reference voltage VC1, while the upper hot reference voltage VH1 can be set a predefined voltage above the lower hot reference voltage VH2. For example, the upper hot reference voltage VH1 can represent a temperature which is 2° C. to 3° C. below the hot trip point temperature.
Further, when the constant current I is input to thermistor 114, the measured battery temperature voltage VB is placed on the measured voltage inputs MM of the cold and hot voltage comparators 110 and 112. Because the constant current I is a 0TC current and the resistance of thermistor 114 varies with temperature, the measured battery temperature voltage VB also varies with temperature, decreasing as the temperature of thermistor 114 increases, and increasing as the temperature of thermistor 114 decreases.
FIG. 2 shows a graph that further illustrates the operation of battery temperature monitoring circuit 100. As shown in FIG. 2, as long as the measured battery temperature voltage VB remains below the upper cold reference voltage VC1 and above the lower hot reference voltage VH2, the temperature of lithium ion battery 116 remains within a safe charging region.
While in the safe charging region, the too cold signal VTC and the too hot signal VTH are both output with safe logic states. For example, the too cold signal VTC can represent a safe charging condition with a logic low, while the too hot signal VTH can represent a safe charging condition with a logic high.
In addition, cold voltage comparator 110 compares the measured battery temperature voltage VB to the upper cold reference voltage VC1 when the too cold signal VTC has the safe logic state, and changes the safe logic state of the too cold signal VTC to an unsafe logic state, such as a logic high, when the measured battery temperature voltage VB exceeds the upper cold reference voltage VC1.
Thus, when the measured battery temperature voltage VB across thermistor 114 rises above the upper cold reference voltage VC1, which indicates that lithium ion battery 116 is too cold to safely charge, cold voltage comparator 110 trips and changes the logic state of the too cold signal VTC. The battery charging circuit responds to the change in logic state of the too cold signal VTC, and stops charging lithium ion battery 116.
In addition to tripping and changing the logic state of the too cold signal VTC when the measured battery temperature voltage VB rises above the upper cold reference voltage VC1, cold voltage comparator 110 also changes reference voltages, switching out the upper cold reference voltage VC1 and switching in the lower cold reference voltage VC2.
Following this, cold voltage comparator 110 compares the measured battery temperature voltage VB to the lower cold reference voltage VC2, and changes the unsafe logic state of the too cold signal VTC back to the safe logic state when the measured battery temperature voltage VB falls below the lower cold reference voltage VC2.
Thus, when the measured battery temperature voltage VB across thermistor 114 falls below the lower cold reference voltage VC2, cold voltage comparator 110 trips and changes the logic state of the too cold signal VTC. The battery charging circuit responds to the change in logic state of the too cold signal VTC, and begins charging lithium ion battery 116 when all of the remaining conditions for charging have been satisfied.
In addition to tripping and changing the logic state of the too cold signal VTC when the measured battery temperature voltage VB falls below the lower cold reference voltage VC2, cold voltage comparator 110 also changes reference voltages, switching out the lower cold reference voltage VC2 and switching back in the upper cold reference voltage VC1.
On the other hand, hot voltage comparator 112 compares the measured battery temperature voltage VB to the lower hot reference voltage VH2 when the too hot signal VTH has the safe logic state, and changes the safe logic state of the too hot signal VTH to an unsafe logic state, such as a logic low, when the measured battery temperature voltage VB falls below the lower hot reference voltage VH2.
Thus, when the measured battery temperature voltage VB across thermistor 114 falls below the lower hot reference voltage VH2, which indicates that lithium ion battery 116 is too hot to safely charge, hot voltage comparator 112 trips and changes the logic state of the too hot signal VTH. The battery charging circuit responds to the change in logic state of the too hot signal VTH, and stops charging lithium ion battery 116.
In addition to tripping and changing the logic state of the too hot signal VTH when the measured battery temperature voltage VB falls below the lower hot reference voltage VH2, hot voltage comparator 112 also changes reference voltages, switching out the lower hot reference voltage VH2 and switching in the upper hot reference voltage VH1.
Following this, hot voltage comparator 112 compares the measured battery temperature voltage VB to the upper hot reference voltage VH1, and changes the unsafe logic state of the too hot signal VTH back to the safe logic state when the measured battery temperature voltage VB rises above the upper hot reference voltage VH1.
Thus, when the measured battery temperature voltage VB across thermistor 114 rises above the upper hot reference voltage VH1, hot voltage comparator 112 trips and changes the logic state of the too hot signal VTH. The battery charging circuit responds to the change in logic state of the too hot signal VTH, and begins charging lithium ion battery 116 when all of the remaining conditions for charging have been satisfied.
In addition to tripping and changing the logic state of the too hot signal VTH when the measured battery temperature voltage VB rises above the upper hot reference voltage VH1, hot voltage comparator 112 also changes reference voltages, switching out the upper hot reference voltage VH1 and switching back in the lower hot reference voltage VH2.
The voltage comparator circuits 110 and 112 utilize the lower cold reference voltage VC2 and the upper hot reference voltage VH1 for hysteresis to prevent the too cold and too hot signals VTC and VTH from toggling between the safe and unsafe logic states (an undesirable condition which can occur when noise on the measured battery temperature voltage VB causes the measured battery temperature voltage VB to bounce around the upper cold reference voltage VC1 or the lower hot reference voltage VH2).
One problem with battery temperature monitoring circuit 100 is that the input offset voltages of the cold and hot voltage comparators 110 and 112, the error or differences between the magnitude of the actual upper cold reference voltage VC1 and the specified magnitude for the upper cold reference voltage VC1, the error or differences between the magnitude of the actual lower hot reference voltage VH2 and the specified magnitude for the lower hot reference voltage VH2, and the error or differences between the magnitude of the actual constant current I and the specified magnitude for the constant current I, which are introduced by random variations in the manufacturing process, limit the accuracy of circuit 100.
The effect of the input offset voltage of a voltage comparator can be compensated for by increasing the size and silicon footprint of the voltage comparator, but the increase in size and silicon footprint is significant. For example, a voltage comparator with a maximum input offset voltage of 0.5 mV can consume approximately three times more silicon surface area than a voltage comparator with a maximum input offset voltage of 1 mV.
The errors in the magnitudes of the upper cold reference voltage VC1 and the lower hot reference voltage VH2 can be compensated for by trimming the cold and hot reference voltages VC1 and VH2, but trimming the cold and hot reference voltages VC1 and VH2 is a non-trivial matter in terms of accuracy and linearity. Further, the error in the magnitude of the constant current I can be compensated for by trimming current source 120 so that the magnitude of constant current I matches the specified magnitude.
Thus, there is a need for a battery temperature monitoring circuit that compensates for all sources of error, including the input offset voltages of the voltage comparators, the errors in the magnitudes of the cold and hot reference voltages VC1 and VH2, and the error in the magnitude of the constant current I, without requiring a substantial increase in the size and silicon footprint of a voltage comparator, or non-trivial approaches to generating the reference voltages.